Oil accumulated within a subterranean oil-bearing formation is recovered or produced therefrom through wells, called production wells, drilled into the subterranean formation. A large amount of such oil may be left in the subterranean formations if produced only by primary depletion, i.e., where only formation energy is used to recover the oil. Where the initial formation energy is inadequate or has become depleted, supplemental operations, often referred to as secondary, tertiary, enhanced or post-primary recovery operations, may be employed. In some of these operations, a fluid is injected into the formation by pumping it through one or more injection wells drilled into the formation, oil is displaced within and is moved through the formation, and is produced from one or more production wells drilled into the formation. In a particular recovery operation of this sort, seawater, field water or field brine may be employed as the injection fluid and the operation is referred to as a waterflood. The injection water may be referred to as flooding liquid or flooding water as distinguished from the in situ formation, or connate water. Fluids injected later can be referred to as driving fluids. Although water is the most common, injection and drive fluids can include gaseous fluids such as air, steam, carbon dioxide, and the like.
Although conventional waterflooding is effective in obtaining additional oil from some oil-bearing subterranean formations. In other formations, water may have the tendency to “finger” through an oil-bearing formation and to thus bypass substantial portions thereof. By fingering is meant the development of unstable water stream fronts which advance toward the production wells more rapidly than the remainder of the flooding water. Furthermore, when fingering is encountered, the water does not normally displace as much oil in the portions of the formations which it contacts as it is potentially capable of displacing.
Also, waterfloods may be less effective with the more viscous oils than with relatively nonviscous oils. The fingering and bypassing tendencies of water may be related to the ratio of the viscosity of the oil to the viscosity of the flooding water, and also related to fractures and/or high permeability zones in the formation. The viscosity of these oils varies from as low as about one or two centipoise to about 1,000 centipoise or higher. Water generally has a viscosity of about 1 centipoise at room temperature.
In order to restrict the mobility of the flooding water to no greater than the mobility of the oil, water thickening agents have been added to increase the viscosity of the water.
There are two principal mechanisms of enhancing the oil recovery of an injected fluid. These methods include increasing volumetric sweep efficiency of the injected fluid and increasing the oil displacement efficiency by the injected fluid. Both techniques may involve the addition of agents which modify the properties of the injected fluid.
Water may be injected by itself, or as a component of miscible or immiscible displacement fluids. Sea water (for offshore wells) and brine produced from the same or nearby formations (for onshore wells) may be most commonly used as the water source.
Some injection drive fluids include water and a small amount of a water-soluble polymer, such as a polyacrylamide.
Referring to FIG. 1, there is illustrated prior art system 100. System 100 includes body of water 102, underground formation 104, underground formation 106, and underground formation 108. Production facility 110 may be provided at the surface of body of water 102. Well 112 traverses body of water 102 and formation 104, and has openings in formation 106. A portion of formation 106 may be fractured and/or perforated as shown at 114. Oil and gas may be produced from formation 106 through well 112, to production facility 110. Gas and liquid may be separated from each other, gas may be stored in gas storage 116 and liquid may be stored in liquid storage 118.
There is a need in the art for improved systems and methods for producing oil and/or gas from a subterranean formation. In particular, there is a need in the art for systems and methods for providing an improved polymer flood, which achieves a desired viscosity of a flooding fluid.